The crash happened around 7 p.m. Saturday on a rural stretch of Highway 221, between Third and Fourth Line roads, about 55 kilometres south of Montreal.
Sgt. Claude Denis of the Sûreté du Québec described the events leading up to the fatal collision.
He said several cars were driving normally in both directions of the two-lane highway when one driver became impatient.
At a section of the road where the lane is widened to allow for vehicles making left turns, a driver passed the line of traffic on the right – an illegal maneuvre.
"But this lane is not very long, it doesn't go on for kilometres. When the vehicle got to the end of the widened portion, he lost control of the vehicle, hit another vehicle that was travelling in the other direction that had three people inside of it," Denis said.
He said a third vehicle could not avoid the crash and hit one of the two cars.
The offending driver is in a coma, and police say medical authorities fear for his life.
The three people inside the second car have died of their injuries.
Traffic on Highway 221 is being detoured, as police investigators are on the scene to reconstitute the events.
Denis said speed likely played a factor in the fatal collision.